1. Field of the Invention
This invention pertains to the use of a diffraction grating for correcting chromatic dispersion in a Fresnel lens. The primary intended application for the invention would be to improve the optical performance of a Fresnel lens employed as a concentrator element in a high-concentration solar collector.
2. Description of the Prior Art
Chromatic dispersion in an optical system is generally minimized by means of an achromatic doublet comprising two cemented or closely-spaced lens elements composed of optical materials with different dispersive powers. By appropriately balancing the doublet's optical power between the two elements, the combination can be made precisely achromatic at two specific design wavelengths. Color correction at more than two wavelengths can be achieved by using three or more glass materials in the optical system. Such systems exhibit excellent achromatic imaging performance, but this method of dispersion compensation is not practical for many applications due to the high material cost and weight associated with the lens elements and complications associated with multiple-element lens systems. In particular, the method is unsuitable for Fresnel lens systems such as concentrating solar collectors.
A Fresnel lens can be achromatized by means of a dispersion-compensating diffraction grating whose diffraction-induced chromatic dispersion offsets and substantially cancels the lens's refraction-induced dispersion. Two rudimentary systems of this type are known in the prior art: Foster (1958) describes a mechanism comprising two diffraction gratings that function conjunctively to minimize a prism's refraction-induced dispersion. Lohmann (1970) describes the use of a dispersion-compensating grating to offset diffraction-induced dispersion produced by a phase-modulating grating in a phase contrast microscope. Dispersion-compensating mechanisms such as these are only applicable to optical configurations in which the dispersion-producing and dispersion-compensating elements both function merely to deflect a collimated incident beam into a transmitted beam that is also collimated. More general types of dispersion, such as that produced by refraction at a lens surface, cannot be corrected by these means. Furthermore, these systems have the additional disadvantage that the dispersion-compensating mechanism requires extra optical elements which add to the system cost and complexity and reduce its optical transmittance (due to multiple surface reflection losses).